The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the multi-slab acquisition of three-dimensional NMR image data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One method for localizing the region from which NMR signals are acquired to produce an image is referred to in the art as the "slice select" or "slab select" technique. A magnetic field gradient is applied along an axis (e.g. z axis) and a selective RF excitation pulse is applied. The frequencies in the selective RF excitation pulse establish resonance with the spins to be excited at a location along the z axis where the sum of the polarizing magnet field B.sub.0 and the applied gradient field G.sub.z satisfy the condition for Larmor resonance. If the polarizing magnetic field B.sub.0 is homogeneous throughout the region to be imaged, and if the gradient field G.sub.z is perfectly formed, a flat slice, or slab, of spins will be excited. The location of the slice is determined by the center frequency of the selective RF excitation pulse, and its thickness is determined by its bandwidth. Such an ideal slice is shown in FIG. 2.
Because the polarizing magnetic field B.sub.0 is not perfectly homogeneous, in practice the slice select method of spin excitation is less than perfect. Not only are there inhomogeneities in the generated polarizing field B.sub.0, but when an object is placed in the field of view, susceptibility effects further distort the B.sub.0 field, and chemical shift effects come into play. As a result, the excited slice no longer lies in a flat plane, but instead, lies in a complex, contoured plane such as that shown in FIG. 3.
In most MRI pulse sequences the distortion of the selected slice is not a significant problem. However, there are occasions when slice selection is used and then phase encoding is used along the same axis to further localize the acquired NMR signals. Since phase encoding is not dependent on the polarizing magnetic field B.sub.0, the region it selects is not distorted by field inhomogeneities and it will not coincide with the shape of the selected slice.
Such a situation is illustrated in FIG. 4, where three-dimensional NMR data is acquired using a combination of slab selection along the z axis and phase encoding along the same axis. This method is described, for example, in U.S. Pat. No. 4,431,968 entitled "Method of Three-Dimensional NMR Imaging Using Selective Excitation", which is incorporated herein by reference. Due to B.sub.0 field inhomogeneities, the three slabs 10, 11 and 12 are not flat, but instead, are distorted into a complex curved volume. A series of 3DFT acquisitions are made to acquire NMR image data from each excited slab 10-12, and because phase encoding is used to localize along the z axis, the 3D array of NMR data represents samples located in successive, planar slices 14 disposed along the z axis. At the boundaries between slabs 10, 11 and 12, the planar slices 14 of 3D data will not be confined to their excited slab due to the curved slab boundaries. This produces dark bands at the slab boundaries in the reconstructed 3D images, which is known in the art as the "venetian blind" artifact. This artifact is particularly troublesome at high polarizing field strengths and when fast spin echo pulse sequences are used to acquire the 3D NMR data.